Reducing of the thermal conductivity of f-electron Perche materials

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In this study, YbAl_3 was synthesized and the thermoelectric properties were determined. The effect of substitution for Yb and Al sites on the thermoelectric properties was also investigated.Yb lump, Al plate, and powders of Tm, Lu, B, In, Cu and Ag were used as the starting materials. The mixed powders were melted and annealed mainly in vacuum. Tm and Lu containing specimens were heat-treated in Ar to prevent the evaporation. The specimens were grinded and rinsed in 1M NaOH solution to remove excess Al. The rinsed powders were hot-pressed at 31.2 MPa and 1123 K in Ar. The phase and lattice constant were examined by X-ray diffraction. The Seebeck coefficient (α) was determined from the linear relationship between thermoelectromotive force and temperature difference. The electrical resistivity (ρ) and the thermal conductivity (κ) were measured by a dc four-probe method and a laser flash method, respectively. The measurements were conducted at temperature (T) from 100 to 800 K. The power … More factor and dimensionless figure-of-merit (ZT) were evaluated from the equations of α^2/ρ and α^2/ρκ, respectively. The ρ of YbAl_3 synthesized in this study was almost in agreement with the literature values prepared by solid state reaction and arc melting. Although the literature values showed maximum around 200 K, present values of |α| increased with decreasing temperature. The greatest value was 116 μVK^<-1> at 108 K. The power factor also increased with decreasing temperature, being 797×10^<-4> Wm^<-1> K^<-2> at 118 K. This value was 3.5 times as large as reported values of YbAl_3, and was 17 times as large as that of Bi_2Te_3. The temperature dependence of κ for YbAl_3 was first determined in this study. The ZT increased with decreasing temperature, and was 0.29 at 307 K. The Yb sites were substituted with Tm and Lu up to 0.3 mol%. The substitution caused the decrease of ρ and |α|. This suggests that the substitution decreased the scattering of conduction electrons by 4f^<13> level of Yb^<3+>. The Al sites of YbAl_3 were not substituted by B, Cu and Ag. It is confirmed that In substitutes the Al sites up to 0.05mol%. The ρ of YbAl_<2.95>In_<0.05> was almost in agreement with that of YbAl_3, but |α|al decreased by the substitution. Less